Physico-chemical characteristics of evaporating respiratory fluid droplets

TLDR

The physico‑chemical properties of respiratory droplets in ambient air are poorly understood, and changes in the concentration and phase of key components such as salt, protein, and surfactant may influence pathogen viability and transmission. The study aims to determine how relative humidity affects the physico‑chemical characteristics of evaporating droplets of model respiratory fluids. Researchers labeled the major components in model fluids and monitored evaporating droplets on a superhydrophobic surface using optical and fluorescence microscopy. As relative humidity decreased, droplets displayed distinct morphologies, phase separation, pH decline, and surfactant‑mediated inhibition of rapid rehydration, while a surrogate influenza virus remained uniformly distributed, indicating that increasing acidity and salinity during evaporation could alter viral structure, though crystallization at low humidity may reduce these effects.

Abstract

The detailed physico-chemical characteristics of respiratory droplets in ambient air, where they are subject to evaporation, are poorly understood. Changes in the concentration and phase of major components in a droplet—salt (NaCl), protein (mucin) and surfactant (dipalmitoylphosphatidylcholine)—may affect the viability of any pathogens contained within it and thus may affect the efficiency of transmission of infectious disease by droplets and aerosols. The objective of this study is to investigate the effect of relative humidity (RH) on the physico-chemical characteristics of evaporating droplets of model respiratory fluids. We labelled these components in model respiratory fluids and observed evaporating droplets suspended on a superhydrophobic surface using optical and fluorescence microscopy. When exposed to continuously decreasing RH, droplets of different model respiratory fluids assumed different morphologies. Loss of water induced phase separation as well as indication of a decrease in pH. The presence of surfactant inhibited the rapid rehydration of the non-volatile components. An enveloped virus, ϕ 6, that has been proposed as a surrogate for influenza virus appeared to be homogeneously distributed throughout the dried droplet. We hypothesize that the increasing acidity and salinity in evaporating respiratory droplets may affect the structure of the virus, although at low enough RH, crystallization of the droplet components may eliminate their harmful effects.